This invention relates generally to thermally-insulated containers for storing or shipping liquified gases at cryogenic temperatures and at atmospheric pressure, and more particularly to a cryogenic container provided with a prefabricated inner bladder whose configuration roughly conforms to the contours of the inner walls of the container and yet is capable of sustaining the liquid load without rupture.
While a container in accordance with the invention will be described in connection with liquified natural gas (LNG), it is to be understood that the container is also useful for the storage and transportation of other cryogenic liquified gases such as liquified petroleum gas (LPG), ethylene, liquified oxygen and liquified nitrogen.
The rising demand for methane or natural gas is greatest in those highly industrial countries, such as the U.S., Western Europe and Japan, which are deficient in this natural resource. In recent years, it has become the practice to liquify methane at its source and to transport the extremely cold liquified gas at atmospheric pressure to the consumer site where it must be stored.
The fact that natural gas in liquified form occupies a volume that is only one six-hundredth of the fuel in its gaseous state renders the liquefaction process economically feasible even when the liquid must be transported for thousands of miles from an oil field in Africa, the Persian Gulf or Indonesia, where it is readily available to the remote consumer market. To this end, ocean-going vessels have been specifically fitted with cryogenic containers to carry LNG cargoes.
Most LNG containers designed for transoceanic transport are of the free-standing tank or of the membrane tank type. In the usual free-standing tank arrangement, the tank rests on structural insulation material such as composite panels made of balsa wood and plywood, with non-structural insulation filling the non-loaded area. Similar thermal insulation is provided between the upstanding tank walls and the bulkhead or inner hull. Because the free-standing tank must carry a considerable liquid load and is in direct contact with the cryogenic liquid, it must be fabricated of heavy-gauge metals such as aluminum or stainless steel which are capable of carrying the load and are not subject to embrittlement and failure at cryogenic temperatures.
The membrane tank, usually formed of thin metal sheets of nickel alloy steel or material having similar properties, is supported both on the bottom and side walls by structural insulation which is attached to or supported by the ship's bulkhead or inner hull. A membrane tank of this type is disclosed in the Kohn et al. U.S. Pat. No. 3,325,037 wherein a thin metal tank is supported within a thermal insulating structure constituted by balsa-wood sandwich panels of exceptionally high structural strength. Inasmuch as a cryogenic container in accordance with the invention preferably makes use of similar insulation having structural properties, the entire disclosure of this patent is incorporated herein by reference.
In designing a cryogenic container, one must take into account the large differential expansion of the various components of the tank and ship during actual service. The extremes of temperature to which the cryogenic container are subjected will be appreciated when it is realized the liquid hydrocarbons at atmospheric pressure have a temperature of about -258.degree. F, whereas ambient temperature may range between 0.degree. F and +115.degree. F.
There are several known ways by which one may impart characteristics to the walls of the membrane tank which permit these walls to resist dimensional variations as a result of extreme temperature differences without sustaining damage. Thus the walls of the tank may be made up of a welded assembly of corrugated metal plates or flat plates connected together with metallic bellows elements, the metal walls being made integral with an insulating layer.
Metal tanks of the free-standing or membrane type, particularly those of the stainless steel and aluminum alloy variety, tend to be quite costly. Moreover, the intricate expedient heretofore employed to accommodate the tank structure to extreme changes in temperature and to minimize the transmission of stresses between the inner tank and the insulation due to contraction add considerably to the expenses of producing and installing the container.
With a view to reducing the cost of cryogenic containers, the Cuneo U.S. Pat. No. 3,566,524 provides a steel-reinforced concrete tank having a liquid and gas-impervious liner of polyethylene at its inner wall. Inasmuch as this liner has little structural strength, it is vital that the liner conform intimately to the contours of the inner surface of the concrete tank, for otherwise should spaces exist between the polyethylene film and the tank surface, the unsupported load imposed by the cryogenic liquid on the liner will cause rupture thereof.
Hence though a polyethylene liner is less expensive than a metal membrane tank in terms of material costs, the expenses involved in producing and installing a perfectly contoured polyethylene liner are considerable and offset to a large degree the savings in material costs.
Similarly, in the Alleaume U.S. Pat. No. 3,273,373, a cryogenic tank is provided with a liner formed of a homogeneous, flexible and elastic material which, though it serves as a primary barrier, lacks structural properties and is incapable of physically supporting a heavy liquid load.
For membrane tanks, government regulations now require both a primary and secondary barrier layer to ensure that the liquid methane makes no contact with the ship's hull or bulkhead; for should the extremely cold liquid penetrate the primary barrier and find its way to the relatively warm metal of the hull or bulkhead, it will embrittle and fracture this metal. The primary barrier layer must be designed to securely contain the LNG or other cryogenic liquid, whereas the secondary barrier acts as a safety factor in the event of a failure in the primary barrier.
Thus while various forms of cryogenic containers have heretofore been proposed employing as a primary barrier an inner liner of Mylar, fiberglass or other non-metallic material, in all such containers it is essential that this liner which lacks structural properties and is incapable of supporting the load be in intimate contact with the inner wall of the insulation layer so that the liner is backed up throughout its entire area. The existence of any irregularity between the liner and the inner wall cannot be tolerated for a discontinuity at any given point will deprive the liner of its backing and may result in a rupture thereof having serious consequences.